/*
 * Alloy Analyzer 4 -- Copyright (c) 2006-2008, Felix Chang
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

package edu.mit.csail.sdg.alloy4;

import java.io.Serializable;

/**
 * Immutable; stores a pair of object references; Pair.equals() compares by value.
 *
 * <p><b>Thread Safety:</b>  Safe (since objects of this class are immutable).
 */

public final class Pair<A,B> implements Serializable {

    /** This ensures the class can be serialized reliably. */
    private static final long serialVersionUID = 1L;

    /** The first half of the pair. */
    public final A a;

    /** The second half of the pair. */
    public final B b;

    /** Constructs a new Pair object (a,b). */
    public Pair(A a, B b) {
        this.a = a;
        this.b = b;
    }

    /** If either or both "a" and "b" are String, we'll return it as-is; otherwise, we call toString() on them. */
    @Override public String toString() {
        if (a instanceof String) {
            if (b instanceof String) return ((String)a)+" "+b;
            return (String)a;
        }
        if (b instanceof String) {
            return (String)b;
        }
        if (a==null) {
            return (b!=null) ? b.toString() : "";
        } else {
            return (b!=null) ? (a.toString()+" "+b) : a.toString();
        }
    }

    /** Returns a hashcode based on (a==null?0:a.hashCode()) and (b==null?0:b.hashCode()). */
    @Override public int hashCode() {
        int i = (a==null) ? 0 : a.hashCode();
        int j = (b==null) ? 0 : b.hashCode();
        return i*173123+j;
    }

    /** Pairs (a1,b1) and (a2,b2) are equal iff a1.equals(a2) and b1.equals(b2). */
    @Override public boolean equals(Object that) {
        if (this==that) return true;
        if (!(that instanceof Pair)) return false;
        Pair<?,?> p=(Pair<?,?>)that;
        return (a==null ? p.a==null : a.equals(p.a)) && (b==null ? p.b==null : b.equals(p.b));
    }
}
